Multi-User (MU)-MIMO is an important means to increase peak data rate and spectral efficiency in the current wireless communication. To facilitate MU-MIMO operation, advanced feedback schemes are being considered in 4G wireless standards, such as LTE-A. One of those advanced feedback schemes is BCI (best companion PMI (pre-coding matrix index)) reporting.
In LTE Rel-8, PMI is adopted for Single-User (SU)-MIMO, i.e., the UE reports the most preferred pre-coding matrix to be used for the UE itself. PMI reporting is useful. Further, with BCI reporting, the UE also reports the most preferred pre-coding matrix to be used for the possibly co-scheduled UE.
FIG. 1 shows a schematic diagram that the UE feedbacks to a base station the most preferred PMI to be used for itself and the most preferred BCI to be used for possibly co-scheduled UE. As shown in the FIG. 1, a wireless communication system 100 may comprise a mobile terminal (UE) 101 and a base station 103, in which the mobile terminal 101 reports to the base station 103 the most preferred PMI to be used for itself and the most preferred BCI to be used for a possibly co-scheduled UE (another mobile terminal) 102. Similar to the PMI/CQI reporting pair, BCI reporting can be accompanied by a delta CQI, which indicates the CQI degradation due to the co-scheduled UE that adopts the reported BCI as its PMI.
In the LTE-advanced standardization, a double codebook structure for the PMI reporting of 8Tx antenna (eNB-base station) is agreed. The rank one pro-coding vector of the double codebook is in the form of [viT k·viT]T, where i ε {0, 1, 2, . . . 31}, and k ε {1, −1, j, −j}. The vector vi is a 4×1 column vector, which can be expressed as:
            v      i        =          [                                    1                                                              exp              ⁡                              (                                  j                  ⁢                                                                                    ⅈ                        ·                        2                                            ⁢                      π                                        32                                                  )                                                                                        exp              ⁡                              (                                  j                  ⁢                                                                                    2                        ·                        ⅈ                        ·                        2                                            ⁢                      π                                        32                                                  )                                                                                        exp              ⁡                              (                                  j                  ⁢                                                                                    3                        ·                        ⅈ                        ·                        2                                            ⁢                      π                                        32                                                  )                                                        ]        ,where j denotes the imaginary unit.
FIG. 2 shows a diagram of the agreed double codebook structure of the rank one used for the PMI reporting of 8Tx antenna. It is seen that the rank one codebook represents the combination of DFT (Discrete Fourier Transform) beam (component) and co-phasing factors. However, the reporting of DFT beam index and co-phasing factors is not to simply report the above two things. Instead, a two-level report corresponding to the double codebook is adopted: the UE firstly feedbacks a rough knowledge of PMI (a long-term/wideband component, W1), which indicates to the base station that possible pre-coding vectors are in a DFT direction {0, 1, 2, 3} when W1=0, and the co-phasing factors can be {1, −1, j, −j}. W1 is a four-bit signal because there is overlapping of the possible pre-coding vectors between {W1=0} and {W1=1}. After W1 is reported, the UE further reports another more accurate knowledge of PMI (a short-term/narrowband component, W2), which indicates to the base station that the exact pre-coding vector is in the set confined by W1. In the FIG. 2, the possible DFT beam indexes in case of W1=0 is from v0 to v3, the possible DFT beam indexes in case of W1=1 is from v2 to v5, the possible pre-coding vectors when W1=2 is from v4 to v7, . . . , the possible DFT beam indexes in case of W1=14 is from v28 to v31, and the possible DFT bean indexes in case of W1=15 includes v30, v31, V0, and V1.
The advantage of the double codebook is to better exploit the continuity of the pre-coding vectors in the time/frequency domain, so either the overhead can be improved comparing with a 7-bit single codebook design without significant loss of reporting accuracy.
An important feature of the agreed double codebook is that the adjacent DFT beams (in terms of index) are also adjacent in terms of their directionality.
FIG. 3 shows a diagram of the directionality of the DFT beams.
As shown in the FIG. 3, when w1=0, the adjacent DFT beams v0, v1, v2, v3 are also adjacent in their directionality.
Because the BCI is calculated based on the PMI assumption, how to report the BCI in conjunction with the double codebook based PMI is a problem.